Learning about Xbee or Zigbee protocols can be made easier if one follows Robert Faludi’s blog / book based on Wireless Sensor Networks. It is explained in great detail and is a pleasure to read! The book can be downloaded from here. The blog never ceases to amuse me with its new ideas!

Learning OpenCV -> Ebook by Gary Bradski. One of the best books in the market. Apart from this, we also have Willowgarage wiki page.